For controlling the energy supplied to the downstream electric motor, the switching device comprises a control unit and a first current path. Via the first current path, a first phase of the power supply network is conducted to the downstream electric motor. The first current path comprises a first electromechanical switch and a parallel circuit of a second electromechanical switch with a semiconductor switch connected in series to the first switch. The control unit may emit a switching signal for the first switch, the second switch and the semiconductor switch and thereby control the desired switching state of the switches. The first and second switches are, in particular, normally open contacts which are held in the closed switching state when a switching signal of the control unit is present. If the switching signal from the first or second switch is switched off, the switch automatically adopts the open state due to a restoring force (for example a spring force which acts on a contact of the respective switch). Such switches are, in particular, relays. The semiconductor switch preferably adopts the electrically conductive state in the presence of the switching signal of the control unit.
The switching device comprises a power supply connection via which the control unit is able to obtain the energy for the switching signals. Conventionally, a decentralized power supply source is connected by way of a conductor to the power supply connection of the switching device, so that in active operation of the switching device a supply voltage is applied via the power supply connection. The switching device obtains the energy required for the switching signals via the power supply connection. Preferably, the entire energy supply of the switching device inside the device is obtained via the power supply connection of the switching device.
The switching device is preferably used in order to switch on and switch off three-phase motors and also single-phase alternating current motors. The motors arranged downstream of the switching device are preferably also protected by the switching device against overload (short circuit protection and/or thermal overload).
If the switching device is used in applications which are critical in terms of safety, a safe disconnection of the electric motor arranged downstream of the switching device has to be ensured by the switching device.
In switching devices with emergency shutdown of the motor arranged downstream of the switching device, one possible disconnection principle is to disconnect the supply voltage of the switching device by way of a switching device (for example an emergency stop switching device) connected in series in the supply section between the power supply source and the power supply connection. Due to their mode of operation, by disconnecting the supply voltage the electromechanical switches of the switching device fall automatically into the off state (i.e. the switches are open). In this manner, when the supply voltage is switched off, the energy supply conducted to the motor via the switching device is disconnected so that the motor is safely switched off. By switching off the power supply source and automatically opening the switches, a switch-off arc is formed on the switches which causes a high degree of wear on the switches so that the number of such switching operations is generally limited in the switching devices.